Spatial distribution of δ18O in rainwater and groundwater to identify areas of recharge in the Colombian Northwest

The understanding of the spatiotemporal variability of the water molecule stable isotopes (δ18O and δ2H) in rain and groundwater has been used in hydrogeology to validate recharge zones. In this study, we analyzed the recharge of four aquifer systems located in northwestern Colombia. This region is characterized by highly complex topography, as the Andes Mountain range splits into three branches, two of them included in the study area. The relation in the variation of values of the δ18O‰ between Precipitation (P) and groundwater (GW) was analyzed through the arithmetic expression P/GW; when P/GW is equal or larger than 1, the recharge is direct, and values less than 1 indicate recharge from regional flows. For the purposes of this research, according to statistics criteria, values between 0.98 and 1.02 are considered as 1. It was found that on Bajo Cauca, Occidente and Urabá, the phreatic aquifers are recharged directly with rainwater or after slight evaporation processes, while the recharge of deep aquifers occur through regional flows. The Valle de Aburrá´s phreatic aquifer is also recharged from distant areas; this occurs because the surface has been impermeabilized by urban processes. The P/GW ratio seems to be useful in identifying recharge processes in regions with higher elevation gradients. In low-lying areas, the applicability of this method should be restricted.